1. Field of the Invention
The present invention relates to an optical add/drop device that can add/drop an optical signal having an arbitrary wavelength in an optical communications appliance used in an optical communications network.
2. Description of the Related Art
The demand for an ultra-long-haul and large-capacity optical communications device has been growing for the purpose of configuring a future multimedia network. As a method implementing a large capacity, a WDM (Wavelength Division Multiplexing) method exists. Research and development of this method have been under way since there are advantages such that an optical bandwidth/large capacity of an optical fiber can be effectively used, and the like.
Especially, in an optical communications network, a function for passing/adding/dropping an optical signal on demand, and an optical routing/crossconnect function for selecting an optical transmission path are required at each point on the network. Accordingly, an OADM (Optical Add/Drop Multiplexing) device for passing/adding/dropping an optical signal has been researched and developed. The OADM device is classified into an OADM device of a fixed wavelength type, which can add/drop only an optical signal having a fixed wavelength, and an OADM device of an arbitrary wavelength, which can add/drop an optical signal having an arbitrary wavelength.
In the meantime, an AOTF (Acousto-Optic Tunable filter) operates to extract only light having a selected wavelength. Therefore, the AOTF can arbitrarily select a wavelength unlike a fiber grating whose selected wavelength is fixed. Since the AOTF is also a variable wavelength selecting filter, it is also available as a variable wavelength selecting filter in a tributary station, which is a station for adding/dropping an optical signal between end stations. For such reasons, an OADM device using the AOTF has been researched and developed.
FIG. 1 shows a configuration for explaining the principle of operations of the AOTF. In this figure, the AOTF is configured by forming two optical waveguides 1 and 2 on a substrate 7 of lithium niobate (LiNbO3), which is one type of ferroelectric crystals and shows a piezoelectric effect, with a titanium (Ti) diffusion method.
These optical waveguides 1 and 2 intersect in two portions, and PBSs (Polarization Beam Splitters) 3 and 4 of a waveguide type are provided in the portions where the waveguides intersect. Additionally, a SAW guide 6 of a metal film is formed on the two optical waveguides 1 and 2 between the portions where the two waveguides 1 and 2 intersect. For the SAW guide 6, an IDT (Inter Digital Transducer) 5 having an interleaved comb structure is provided. A SAW (Surface Acoustic Wave), which is generated by applying a control signal (hereinafter referred to as an RF signal) of a 160- to 180-MHz band to the IDT 5, propagates along the SAW guide 6.
In FIG. 1, if light beams having wavelengths λ1, λ2, and λ3 are input to a port 11 of the AOTF, the input light where polarization wave modes such as TE mode and TM mode are mixed is separated by the PBS 3 into the TE mode and the TM mode, and the TE mode and the TM mode light beams propagate along the optical waveguides 1 and 2. Here, a surface acoustic wave is generated by applying an RF signal f1 having a particular frequency, which is generated by an RF signal generating circuit 10, to the IDT 5. If the surface acoustic wave propagates along the SAW guide 6, refractive indices of the two optical waveguides 1 and 2 periodically change due to an AO (Acousto-Optic) effect in portions of the optical waveguides 1 and 2, which intersect the SAW guide 6.
Accordingly, polarization wave modes rotate only in light having a particular wavelength, which interacts with the periodical change in the refractive indices, in the input light, and the TE mode and the TM mode change their places. A rotation amount is proportional to an action length with which the light beams of the TE and the TM modes interact with the change in the refractive indices, and the power of an RF signal. The action length is adjusted by an interval between absorbing elements 8 and 9, which sandwich the IDT 5 and are formed on the optical waveguides 1 and 2, and absorb a surface acoustic wave. Accordingly, the action length and the power of an RF signal are optimized, whereby the TM mode light having that wavelength is converted into the TE mode light on the optical waveguide 1, and TE mode light having that wavelength is converted into TM mode light on the optical waveguide 2. Then, traveling directions of the converted TE mode light and TM mode light are changed by the PBS4. As a result, only light having a wavelength which makes interaction is selected as dropped light, and light having a wavelength which does not make interaction passes through, and becomes output light.
FIG. 1 shows that the optical signals having the wavelength λ1 are acted upon by the RF signal f1, and selected as dropped light.
As described above, the AOTF can select and drop only light having a wavelength according to the frequency of an RF signal. Additionally, the AOTF can change the wavelength of selected light by varying the frequency of the RF signal.
Additionally, at this time, light beams output from a port 12 are optical signals (wavelengths λ2 and λ3), which are obtained by removing only light having a wavelength corresponding to the frequency of an RF signal from the light beams input to the port 11. Therefore, the AOTF can be considered to have a rejection function.
FIG. 2 is a schematic for explaining a configuration of a drop-type AOTF.
The drop-type AOTF shown in this figure has a structure where two types of side-lobe reducing methods are used together, and is configured by thin-film skew SAW guides connected in two stages. This AOTF can select one wavelength at high extinction ratio and with a low loss. A characteristic of an add loss of 3 dB or smaller is implemented with a high extinction ratio of −25 dB or smaller for adjacent side-lobes having an interval of 100 GHz, and −35 dB or smaller for non-adjacent side-lobes. This reaches a practical level. A change in a selected wavelength has a linear relationship with a change in the frequency of an RF signal. It was verified that switching can be quickly made for a high speed of 12 μs including a switching signal delay within a control circuit.
In FIG. 2, the AOTFs which drop an optical signal having a desired wavelength are only connected in two stages in series, and the principle of their operations is similar to that shown in FIG. 1. Therefore, corresponding portions are only denoted with corresponding numerals, and an explanation of their operations is omitted.
FIG. 3 explains a configuration of a 5-ch integrated drop-type AOTF module.
As shown in this figure, the AOTF is a waveguide-type device. Therefore, it is easy to put AOTFs into an array and to integrate the AOTFs. Although this figure shows an example using the 5-ch integrated drop-type AOTF, the AOTF can be implemented as an AOTF having more channels. This module is configured by a 5-ch integrated drop-type AOTF 20, an RF signal generating circuit 21, a signal processing circuit 22, and an optical monitor circuit 23. This AOTF can simultaneously select arbitrary 4 wavelengths from a WDM signal having an interval of 100 GHz. Selected light beams from the AOTF are branched by optical taps 24, PD monitor values are obtained the an optical monitor circuit 23, and arithmetic operations are performed by the signal processing circuit 22 based on the PD monitor values, so that the RF signal generating circuit 21 is controlled. A wavelength search at the time of a wavelength selection, and RF frequency tracking, which matches an AOTF transmission characteristic after the selection with a signal wavelength, are controlled by the signal processing circuit 22 and firmware. A fifth channel on the same substrate is available as a reference for a selection as shown in FIG. 3. Since influences of a deviation caused by temperature or environments of a component, etc., can be eliminated in this way, the accuracy of control can be improved by using a relationship between the wavelength of the reference light source selected by the fifth channel AOTF and an RF signal for other channels' control.
FIG. 4 explains a configuration of a reject-type AOTF.
If an unselected light output port of the AOTF is used, it functions as a reject-type filter, which blocks a particular wavelength.
To implement a reject ratio of a high practical level, the reject-type AOTF has a configuration where 3 AOTFs having the same characteristic are connected in 3 stages by being folded at end faces with waveguide-type reflectors in FIG. 4. By applying an RF signal, different wavelengths having a wavelength interval of 200 GHz can be simultaneously rejected. However, since the AOTFs are connected in 3 stages, its add loss is larger than that of the drop-type AOTF connected in 2 stages in FIG. 2.
In FIG. 4, the reject-type AOTF performs operations similar to those explained with reference to FIG. 1 except that the AOTFs are connected in 3 stages in series by using the unselected light output port of the AOTF. Therefore, an explanation of its operations is omitted.
FIG. 5 explains the operations of the reject-type AOTF module.
The AOTF module shown in FIG. 5 has a configuration where 4 wavelengths can be rejected. To recognize light of a WDM signal which flows from a network beforehand, a drop-type AOTF 31 is configured on the same substrate as the reject-type AOTF 30, the frequency of an RF signal output from the RF signal generating circuit 32 is changed from 160 MHz to 180 MHz, and the spectrum of selected light is monitored with the selected light monitor circuit 32. From monitoring results, RF signals corresponding to wavelengths desired to be rejected are generated with RF signal generating circuits 33-1 to 33-4, and given to the reject-type AOTF 30. If 2 wavelengths are simultaneously rejected, frequencies of 2 RF signals are generated, mixed with a mixer 34, and applied. If 4 wavelengths are simultaneously rejected, frequencies of 4 RF signals are generated, mixed with the mixer, and applied. An unselected light monitor circuit 35 is a circuit for monitoring whether or not the reject-type AOTF 30 can properly reject a desired wavelength. A signal processing circuit 36 performs the detection and the processes of monitor signals of the selected light monitor circuit 36 and the unselected light monitor circuit 35, and inputs an instruction signal to the RF signal generating circuits 32, and 33-1 to 33-4.
FIG. 6 explains a drawing effect of the reject-type AOTF.
In this figure, (a) shows a WDM signal input to the reject-type AOTF module shown in FIG. 5. 32 C- and L-band wavelengths composed of λ1 to λ32 having a wavelength interval of 200 GHz configure the WDM signal. (b) shows a WDM signal output from the reject-type AOTF module. Dotted lines of λ2 to λ3 indicate a rejected state. (c) shows RF signals given to the reject-type AOTF module in order to reject λ2 and λ3 from the WDM input signal. f2 and f3 respectively correspond to λ2 and λ3. Dotted lines indicate unused RF signals. In (c), a frequency interval of the RF signals is indicated by Δf1. Normally, the frequency interval is approximately 100 kHz. (d) shows a case where λ2 to λ5 are rejected from the WDM input signal. (e) shows RF signals given to the reject-type AOTF module in order to implement (d). (e) shows that the frequency interval of the RF signals changes from Δf1 to Δf2 (Δf2<Δf1) by changing from the rejection of 2 wavelengths to the rejection of 4 wavelengths. The phenomenon that the frequency interval of RF signals changes from Δf1 to Δf2 (Δf2<Δf1) due to an increase in the number of rejected wavelengths is referred to as a “drawing effect” in this specification. The “drawing effect”, which occurs when the number of rejected wavelengths increases, is a cause to make a control for rejecting a wavelength complex, and this effect is one of serious problems in control. It is difficult to apply the reject-type AOTF to an OADM due to the “drawing effect”, and a restriction such that a wavelength rejection interval of a practical level is 200 GHz. Therefore, a fixed-band rejection filter using a dielectric multi-layer film, etc., is used at present. Accordingly, an add function is a fixed type.
FIG. 7 explains a concept of functions of an optical add/drop (OADM) device.
When WDM light including optical wavelengths λ1, λ2, λ3, and λ4 is input to a port a of the optical add/drop device, the wavelengths λ1, λ2, λ3, and λ4, which are dropped within the device, are output to a port c.
Additionally, light is added from a port d. The example shown in FIG. 7 indicates that wavelengths λ5, λ6, λ7, and λ8 are added to the output light of the port b, and output.
Explanation of an OADM Device Using a Reject-Type AOTF
FIG. 8 explains operations of an optical add/drop device using a reject-type AOTF module.
In this figure, the reject-type AOTF module 40 is used to reject a wavelength within network light, which corresponds to an added wavelength, when an adding unit adds the wavelength to the WDM signal which flows from a network. This device has restrictions such as the complexity of control caused by the “drawing effect”, and a practical level of a wavelength rejection interval of 200 GHz. In drop-type AOTF modules 41-1 to 41-4 of a dropping unit can drop a wavelength at a wavelength interval of 100 GHz, and do not have the complexity of control caused by the “drawing effect”.
WDM light input from a port a is configured, for example, by 16 C-band wavelengths and 16 L-band wavelengths. An optical branching coupler 42 simply branches the input WDM light. A WDM amplifier 43 amplifies the dropped WDM light. Then, an optical splitting coupler 44 splits the WDM light into the number of wavelengths to be dropped. Optical signals having wavelengths λ1 to λ4 are output from the WDM light beams input to the drop-type AOTF modules 41-1 to 41-4.
After wavelengths λ5 to λ8 of added light beams are converted by AOTF-type tunable transponders 45-1 to 45-4, which will be described later, and amplified by optical amplifiers 46-1 to 46-4, these wavelengths are coupled by an optical coupling coupler 47, and coupled with through light by an optical coupling coupler 48. At this time, the reject-type AOTF module 40 rejects optical signals having the same wavelengths as those of the added optical signals. WDM light output from the optical coupling coupler 48 is amplified by a WDM amplifier 49, and transmitted to a transmission path.
FIG. 9 explains operations of an optical add/drop device using a fixed-band rejection filter.
In this figure, the same constituent elements as those shown in FIG. 8 are denoted with the same reference numerals, and their explanations are omitted.
With the configuration shown in FIG. 9, problems such as the complexity of control caused by the “drawing effect”, and a restriction of a wavelength rejection interval to 200 GHz at a minimum can be solved. However, since an add function is fixed, flexibility is lost. For example, if an optical network is configured with an OADM device using a fixed-band rejection filter 51, the side of an Add function is fixed to 4 wavelengths. Therefore, the number of nodes is restricted to 16 if the network is configured in consideration of a WDM signal having a maximum of 64 wavelengths.
In the fixed-type optical add/drop device shown in FIG. 9, wavelengths to be added are fixed. Therefore, a wavelength conversion function of transponders 45′-1 to 45′-4 may be fixed. Wavelengths are input to the fixed-band rejection filter 51 via a variable optical attenuator 50, coupled with through light, and transmitted to a transmission path.
In an optical communications network, a shift from a conventional optical stream transmission method to an optical burst switching transmission method is newly anticipated. This is an optical transfer network where the use efficiency of network resources are improved by paying attention to a statistical nature of Internet traffic having a high burst nature, and by assigning a wavelength for a time period required for a burst data transfer at a time interval of the order of milliseconds. By making a shift to the optical burst switching transmission method, the use efficiency of the network resources can be improved. To implement this, wavelength switching of the order of milliseconds or shorter is required. Since an AOTF can make wavelength switching of the order of microseconds, it is known to be effective as a core device of the optical burst switching transmission.
The contents of the above described conventional techniques are recited in the following patent documents 1 to 8, and non-patent documents 1–4.
[patent document 1] Japanese Patent Application Publication No. 2003-344817
[patent document 2] Japanese Patent Application No. 2003-053335
[patent document 3] International Patent Application No. PCT JP 03 04793
[patent document 4] Japanese Patent Application NO. 2003-51741
[patent document 5] Japanese Patent Application Publication No. HEI11-218790
[patent document 6] Japanese Patent Application Publication No. HEI11-289296
[patent document 7] Japanese Patent Application Publication No. 2000-241782
[patent document 8] Japanese Patent Application No. 2003-316973
[non-patent document 1] paper name: “Improving the Speed of Acousto-optic Tunable Filter (AOTF) Control”, paper No. B-10-40, society name: IEICE, 2002 Society Conference, written by M. Noguchi, Y. Kai, T. Ueno, H. Miyata, H. Onaka
[non-patent document 2] paper name: “Recent Technological Advances in AO Elements”, IEICE Transactions C, Vol. J86-C No. 12 pp. 1236 to 1243, written by T. Nakazawa, H. Miyata, H. Miyata, Y. Kai, Y. Tsunoda, H. Onaka
[non-patent document 3] paper name: “Photonic Gateway with Us-order Wavelength Path Control for Metro Access Networks”, society name: ECOC 2003, written by Y. Kai, K. Sone, M. Noguchi, T. Ueno, T. Nakazawa, H. Miyata, H. Miyata, H. Onaka
[non-patent document 4] paper name: “Development of High-speed Wavelegnth Selection Small 4-channel integrated AOTF Subsystem”, paper no. B—10—61, society name: IEICE, 2003 Society Conference, written by M. Noguchi, Y. Kai, T. Ueno, H. Miyata, H. Onaka, T. Nakazawa, H. Miyata
As described above, when light is added in an optical wavelength add/drop device using an AOTF, a wavelength which flows from a network and corresponds to a wavelength to be added must be rejected.
With the conventional techniques, an optical wavelength add/drop device is configured by using a reject-type AOTF. This device has a mechanism such that an RF signal corresponding to a wavelength to be rejected is given to the AOTF. Therefore, a plurality of RF signals must be mixed and given if a plurality of wavelengths are rejected. However, there are problems that the frequency interval of RF signals changes (so-called the drawing effect) due to an increase/decrease in the number of wavelengths, and a control becomes complex.
Conventionally, there is also a problem that a D/U characteristic (a ratio of desirable signals to undesirable signals) deteriorates since RF signals are mixed.
Furthermore, conventionally, a WDM signal must be monitored with a spectrum analyzer function, which hinders high-speed switching from being supported.
Still further, a reject-type AOTF cannot be practically used unless a wavelength interval is equal to or higher than 200 GHz. Due to such problems, a dynamic reject function is not practically used in the present situation, and a fixed-band rejection filter is used. If an optical network is configured with an OADM device using a fixed-band rejection filter, the side of an Add function is fixed to 4 wavelengths. Therefore, the number of nodes is restricted to 16 if the network is configured in consideration of a WDM signal having a maximum of 64 wavelengths.